1. Field of the Invention
The present invention relates to a rack bar and a method for production thereof.
2. Description of the Related Art
The conventional way of forming rack teeth by cold forging alone employed a pipe material with good plastic properties and a special processing machine such as a hydraulic press.
Cold forging to form rack teeth needs six to eight steps and takes a long working time. Moreover, it tends to cause defects to the tip of the formed tooth. This leads to a cost increase and quality variation.
By contrast, hot forging to form rack teeth with rapid heating to 700-800° C. involves low deformation resistance which is one-forth to one-fifth of that involved in cold forging. This results in a good flow in the part being worked, which makes it possible to finish 90-98% by one step of forming, with only one step left for finishing by cold forging. Forming in this way, which is semi-hot forging that takes place immediately before transformation point, prevents growth of crystal grains (due to short heating) and shortens time for oxidation and reduces surface oxidation.
About 90% of commercially available rack bars are currently produced by cutting from solid rod stock. Production of rack bars by cutting from solid rod stock needs more material and higher processing cost as compared with production from pipe stock.
A typical hollow rack bar having an outside diameter of 28 mm and a bore diameter of 20 mm is comparable in strength to a solid rack bar having an outside diameter of 26 mm, both being equal in length.
Tables 1 and 2 show the amount of the raw material and the weight of the finished rack bar for the solid rack bar and the hollow rack bar, respectively.
The solid rack bar is inferior in rack tooth strength to the hollow rack bar because it is formed from a rod stock and hence has its fiber flow parallel to the core, with its teeth cut across its fiber flow, which is undesirable for the bending strength of teeth. Consequently, the solid rack bar needs a large tooth thickness and width.
Japanese Patent Laid-open No. 2003-94140 discloses a method for forming a rack bar from a pipe stock. This method consists of performing plastic working, with a mandrel (harder than the pipe) inserted into the bore of the pipe, so that teeth project outward. This method is subject to such defects as unbalanced fiber flow, surface cracking, surface seizure, and large spring back, which are characteristic of cold plastic working. Prevention or reduction of these defects needs stringent specifications, processing conditions, and processing controls, which leads to an increase in forging steps. This in turn leads to a higher material cost, higher processing cost, and higher quality cost for stable quality assurance in mass production.
On the other hand, Japanese Patent Laid-open No. 2007-253190 discloses a method for producing rack bars from pipe stock by forging from its outside. This method is characterized as follows.    (a) This method employs a steel pipe from which the rack bar is formed. The pipe has a structure imparted by heat treatment which is suitable for cold forging.    (b) After tooth forming, that part of the rack bar opposite to the rack tooth undergoes heat treatment for local hardening. This hardening reduces elongation and bending due to forging load. Unfortunately, this method involves difficulties in accurately forming the back side of the tooth. Any angular displacement would harm the accuracy of rack teeth.    (c) This method employs a core mold harder than the steel pipe in order to form rack teeth.    (d) This method basically relies on cold forging and hence needs a steel pipe with good plasticity (such as the one which has undergone annealing) because cold forging (particularly in the initial stage) encounters a high deformation resistance (which is 2.5 to 4 times that of hot forging). Nevertheless, this method needs local heating or warm working (at 100 to 600° C.) for the tooth forming part. This temperature range is the one in which the steel pipe remains brittle, and hence heating at such temperatures is undesirable for quality assurance because temperature variation causes shrinkage, thereby deteriorating the accuracy of rack teeth.